This invention is directed generally to a fluid control system wherein a fluid motor is utilized for raising and lowering a load. In particular, it is directed to a fluid control system wherein a single-acting hydraulic cylinder is utilized for raising a load and the force of gravity is utilized for lowering the load.
Known systems of this type include a valve having a control valve spool slidable within a bore. Of necessity, a radial operating clearance must be provided to permit relatively free spool movement. Normally, movement of the valve spool to the raise position will direct hydraulic fluid to the lift cylinder so as to raise a load supported thereby. When the lift cylinder is supporting a load in an elevated position, the radial clearance between the valve spool and its associated bore provides a leakage path which may result in downward drift of the load.
One attempt to solve this leakage problem has been to provide a check valve between the lift cylinder and the control valve spool. When the load is to be lowered, it is necessary to cause the check valve to open so that fluid may be directed from the lift cylinder. In some instances, the check valve is vented across the control valve spool. This requires a complicated, precisely machined spool, and again could result in leakage which would cause downward drift of the load.
Often there has been provided some form of fluid pressure responsive pilot means for the check valve. This requires that a source of fluid under pressure be available at all times and, accordingly, leads to excessive energy utilization. Further, where a failure in the control system or a stalled engine, for example, is encountered, the available fluid pressure could be interrupted.